Process and apparatus for pneumatically classifying pulverulent material



May 27, 1969 M. BOUCRAUT ETAL 3,446,355

PROCESS AND APPARATUS FOR PNEUMATICALLY CLASSIFYING PULVERULENT MATERIALv Flled Feb. 28, 1967 Sheet I. of 4 .Qn uh NP May 27, 1969 M. BOUCRAUTEIAL 3,446,355

PROCESS AND APPARATUS FOR PNEUMATIC-ALLY CLASSIFYING PULVERULENTMATERIAL Filed Feb. 28, 1967 Sheet 2 of 4 May 27, 1969 M. BOUCRAUT EI'AL3,446,355

PROCESS AND APPARATUS FOR PNEUMATICALLY CLASSIFYING PULVERULENT MATERIALFiled Feb. 28, 1967 Sheet 3 of 4 Q& a

V a QQN I] Q9 1 QQW 59 Qwk ooh g N m y 27, 1969 v M. BQUCRAUT EI'AL3.446,355

PROCESS AND APPARATUS FOR PNEUMATICALL?! CLASSIFYING PULVERULENTMATERIAL Filed Feb. 28, 1967 sheet 4 'of 4 United States Patent Int.(:1. 1307b 3/02 US. Cl. 209474 16 Claims ABSTRACT OF THE DISCLOSURE Aprocess and apparatus for pneumatically classifying pulverulent materialin which a continuous stream of pulverulent material is fed at one endof an elongated relatively high and narrow receptacle into the latterand in which a stream of gas under pressure is fed with such a speedthrough the perforated bottom of the receptacle to form above theperforated bottom a layer of large size particles in suspension withoutlifting the largest particles out of the layer, While the small sizeparticles are lifted out of the layer into the space above the latter,and in which the larger size particles are continuously discharged fromthe other end of the receptacle while the small size particles aredischarged together with the gas from the top region of the receptacle.

Background of the invention The present invention relates to a processand apparatus for pneumatically classifying pulverulent material, thatis to separate large size particles contained in a mass of pulverulentmaterial from small size particles contained therein.

Various apparatus are known for pneumatically classiing pulverulentmaterial.

It is an object of the present invention to provide for a process and anapparatus for pneumatically classifying pulverulent material in aneflicient continuous process in which the separation of the pulverulentmaterial according to grain size can be carried out with greatprecision.

Summary of the invention With these objects in view, the processaccording to the present invention of pneumatically classifyingpulverulent material mainly comprises the steps of forming an elongatedfluidized bed of pulverulent material of a height not exceeding 30 cm.and of a length at least six times the width of the bed by feeding intothe pulverulent material upwardly directed gas with a speed at leastequal to the fluidizing speed of the largest particles of thepulverulent material and inferior to a speed to lift the largestparticles out of the fluidized bed, but superior to a speed to carryaway the smallest particles, continuously feeding a stream ofpulverulent material to be classified into one end of the elongated bed,continuously withdrawing the largest particles at the opposite end ofthe bed so that the largest particles will move from the one to theother end of the bed, maintaining above the fluidized bed a free spaceat least eight times the height of the fluidized bed into which thefinest particles of the pulverulent material are carried by the upwardlydirected gas, evacuating the gas stream with the finest particlessuspended therein from the space, and separating the finest particlesfrom the gas streams.

The process may also have the following characteristics combined withthe characteristics set forth above:

(a) At least two upwardly directed streams of gas are 3,446,355 PatentedMay 27, 1969 introduced into the bed with one of the streams closer tothe other end of the bed having a higher speed than the other.

(b) A lateral stream of gas is introduced into the stream of pulverulentmaterial fed at the one end into the fluidized bed so as to facilitateremoval of dust clinging to the largest particles.

(c) The upwardly directed gas is fed under an angle inclined indirection toward the other end of the bed to the vertical into the bedto enhance displacement of the largest particles from the one to theother end of the bed.

It is also an object of the present invention to provide for anapparatus for carrying out the above set forth method, which apparatusmainly comprises an elongated receptacle having at least a length sixtimes its width and a perforated bottom Wall, means communicating at oneend of the receptacle and at a distance of 5-30 cm. above the bottomwall with the interior of the receptacle for continuously feeding astream of pulverulent material into the latter, means for feeding a gasunder pressure in upward direction and at the aforementioned speedthrough the perforated bottom wall of the receptacle so as to form fromthe largest particles of the pulverulent material fed thereinto afluidized bed above the bottom wall, means for discharging the largestparticles from the other end of the receptacle, the receptacle havingabove the fluidized bed a height of at least eight times the height ofthe fluidized bed, and means communicating with the top portion of thereceptacle for discharging the gas carrying the finest particles of thepulverulent material from the interior of the receptacle.

The apparatus according to the present invention may also have thefollowing characteristics combined with the characteristics statedabove:

(a) The means for feeding a gas stream in upward direction through theperforated bottom wall into the interior of the receptacle may compriseat least two gas passages, one of which communicates at its inner endwith the interior of the receptacle closer to the other end thereof thanthe other of the two passages, and means for feeding gas streams ofdifferent speeds through the aforementioned two passages.

(b) The apparatus may also include a means for introducing into the airstream of pulverulent material as it is fed into the receptacle a streamof gas under pressure in direction transverse to the stream ofpulverulent material to enhance separation of large size particles ofthe pulverulent material from small size particles at the point ofintroducing the pulverulent material into the receptacle.

(c) The means for feeding gas under pressure through the perforatedbottom wall may extend inclined toward the other end of the receptacleto the vertical so as to enhance flow of the large size particles of thefluidized bed from the one to the other end of the receptacle.

As far as understood, the present invention consists of pneumaticallyclassifying a pulverulent material into distinct fractions according tograin size by suspending the particles in upwardly directed gas streamcirculating in a receptacle in such a manner that the particles of asize superior to a certain limit are concentrated in a layer forming afluidized bed at the bottom region of the receptacle, whereas theparticles having a dimension inferior to the predetermined limit arelifted by the gas stream toward the top of the receptacle. In order todo so, the speed of the upwardly directed gas stream is maintained atsuch a speed that the large size particles are not carried away by thegas stream but only held in suspension, whereas the small size particlesare lifted by the gas stream out of the bed in suspension formed by thelarge size particles.

The process according to the present invention in which the product tobe classified is placed in suspension and in which the finest particlesare lifted by the gas currents, closely resembles the pneumaticseparation process known as elutriation. This process essentiallydiffers from a classification process by fluidization in which thepulverulent material to be classified is fluidized above a grill in afluidized bed through which upwardly directed air streams circulate andin which the speed of the air streams is such that the fluidized mass isprogressively separated into layers of different grain sizes anddensities and in which the particles of different grain size in thevarious sections are separately collected at different levels of thefluidized bed. Since the pulverulent product to be classified alwayscontains a small proportion of very fine dust, there is always, in theclassifying process in which all of the particles are maintained in afluidized bed a small fraction of the particles which are carried awayby the fluidizing gas; however, this peculiarity should not lead toconfuse the classifying process in which the whole mass of particles tobe classified are maintained in a fluidized bed with the process ofclassifying by elutriation accord ing to the present invention, sincethe quantity of dust carried away by the fluidizing gas is relativelysmall, for instance 5% compared to the total mass of pulverulentmaterial contained in the fluidized bed and the size limit of the dustthus carried away will always be uncertain and not precise.

Contrary thereto, the present invention proposes to effectclassification by elutriation by placing the pulverulent material to beclassified in suspension. This placing in suspension according to thepresent invention differs from the fiuidizing according to the prior artby the speed of the gas in the interior of the pulverulent material andby the effect of this gas speed on the product, during fluidization thespeed of the gas streams is such that practically the whole mass ofpulverulent material is fluidized in the bed and only the extremely finedust mixed with the product to be classified is carried away by the gas,whereas in the elutriation process according to the present invention anotable fraction per weight of the product to be classified, forinstance at least 20%, are carried away by the gas streams whereas thelargest particles form a very agitated layer in suspension in which noStratification according to dimensions or densities takes place,

It is an important advantage of the present invention to permit a sharpseparation into fractions of different grain size due to the combinationof the characteristics of the process. In order to realize a sharp andprecise separation according to grain size, it is necessary to give thereceptacle in which the separation takes place the form of an elongatedchannel having a length which is very great as compared to its width andthe ratio of these two dimensions should be at least 6 and preferablyequal to 10. This ratio of the length to width of the receptacle is anessential characteristic according to the present invention since it isabsolutely necessary that the particles to be classified follow asubstantially rectilinear path from the point of their introduction intothe receptacle to the point of discharge therefrom without having thepossibility to return. In the fluidizing receptacles known in the artwhich do not have the aforementioned ratio of dimensions, the particleswill move erratically and in all directions through the receptacle whichin turn will cause a homogenization of the fluidized mass.

In the process according to the present invention, the mass to beclassified flows only in one direction while forming a layer in whichthe particles of larger size are maintained in suspension and from whichthe small particles are gradually exhausted during progression of thematerial along the channel. The progressive exhaustion of the small sizeparticles from the layer does not proceed as a linear function of thedistance travelled by the material along the channel, but theprogressive exhaustion follows rather a logarithmic function a d it istherefore necessary in order to obtain a very good separation accordingto grain size to give the treating receptacle a considerable length, forinstance a plurality of meters since doubling the length of the channeldoes not produce doubling of the perfection of the separation accordingto grain size. In carrying out the method of the present invention, ithas been ascertained that if the width of the receptacle is maderelatively large as compared to its length, the efliciency of theapparatus to properly classify the pulverulent material according tograin size diminishes rapidly which justifies the necessity to form thereceptacle with the width and length relationship as mentioned above andthis feature distinguishes the process according to the presentinvention from similar processes of classification by fluidization knownin the art.

Another important characteristic of the process according to the presentinvention is the thickness of the layer of material which is held insuspension. In order to obtain a good efiiciency of separation accordingto grain size, the thickness of layer of material which is held insuspension has to be relatively small in order not to prevent movementof the particles of small size out of the layer by the upwardly directedgas streams. A too great thickness of the aforementioned layer entailsthe risk to retain a relatively large part of the fine particlesimprisoned in the layer, especially such particles which have a grainsize in the neighborhood of the grain size limit at which a division orseparation of the particles should be attained, which would evidentlydiminish the precision of the desired separation according to grainsize. The most effective thickness of the aforementioned layer willdepend on the material to be classified and it can be exactly determinedin each case by experiments. For finely ground iron ore the thickness ischosen between 5 and 10 cm. and in general the thickness of theaforementioned layer should not exceed 25 cm. It is likewise importantto maintain in the treating receptacle a free space above the layer insuspension which is a multiple of the thickness of the aforementionedlayer, at least eight times the layer thickness and preferably 20 to 30times of the same. In order to obtain an efficient process, it is alsonecessary to maintain a speed of the upwardly directed gas streams muchgreater than in known processes of fluidization. The gas speeds used inthe process according to the present invention are, for example, 1-2meters per seconds, while in processes of fluidization known in the art,gas speeds of 60-70 cm. per second are used. In this way a layer insuspension is obtained which forms a very turbulent bed out of whichsome particles of a grain size superior to the grain size limit at whichseparation is to be made are projected upwardly from the bed due to apseudo-viscosity present in the turbulent bed. These particles arethrown upwardly to a certain height and it is necessary to maintainabove the layer in suspension enough space so as to give these particlesa decisive chance to fall back in the layer of suspension before theyare carried away by the gas stream.

The combination of the above-mentioned essential characteristicsaccording to the present invention permits to obtain an exact separationof pulverulent material according to grain size not obtained so far withpneumatic processes known in the art.

The apparatus according to the present invention comprises a receptaclein the form of a relatively narrow and long passageway having a bottomwall in form of a grate of fluidization over which circulate theproducts to be classified and through which gas streams are passed inorder to maintain the products in suspension. The pulverulent materialto be classified is introduced at one end of the passageway at a pointlocated above the grate so as to communicate with the interior of thereceptacle at a point slightly above the layer in suspension. Theparticles which form the large particle size fraction of the pulverulentmaterial are discharged at the opposite end of the receptacle and movetherefore in horizontal direction above the grate of fluidization. Thelarge size particles are discharged through a discharge opening locateda small distance above the grate, which distance defines in fact thethickness of the layer of suspension. Since as mentioned above thelength of the receptacle is considerable, the separation of theparticles according to their dimension will be very precise, since theparticles while passing horizontally along the length of the receptacleare constantly subjected to the action of the upwardly directed gasstreams.

Due to the intense mixing action in the bed of suspension, the largerparticles or the granular particles are subjected to an especiallyintense dust removal action which contributes to the excellent result ofseparation according to grain size. The granular particles of thepulverulent material are frequently covered with very fine dustparticles which adhere lightly thereto but sufiicient to resist a shortdust removing action. Due to the prolonged passage of these granularparticles in the vertical gas stream, the dust particles adheringthereto are pulled oif and carried away by the upwardly directed gasstream to be evacuated from the interior of the receptacle. According toa further feature according to the present invention this pulling awayof the dust particles is enhanced by the introduction of a lateral airstream into the receptacle in the vicinity of the point of introductionof pulverulent material to be treated in the receptacle.

In order to enhance the regularity of the horizontal movement of thelayer of particles in suspension toward the discharge opening, ahorizontal component is imparted to the gas stream introduced throughthe bottom of the receptacle. This horizontal component may be derivedin various ways which may be used separately or in combination. Thus,gas passages may be provided which communicate through the bottom of thereceptacle inclined at an angle with respect thereto, or the grateconstituting the bottom of the receptacle may be formed by twosuperimposed perforated sheet metal members, the openings of which areslightly displaced in such a manner that the gas streams passing throughthe thus formed grate will follow a path obliquely inclined to thebottom of the receptacle in such a manner so as to direct the granularparticles toward the discharge opening, or it is also possible to use aninclined grate or bars descending toward the discharge opening. Otherknown means may also be used in order to impart to the gas a horizontalcomponent. Due to the oblique path of the gas, the length of the gasstream is somewhat extended which also contributes to an improvedseparation of the granular material into the desired sections accordingto grain size.

The present invention permits also regulation or change of the line ofseparation, that is according to the usual definition, the dimension ofa particle, the chances of which to be classified in the section of thesmall particles are the same as the chances to be classified in thesection of large particles. By increasing the speed of the gas streamthe line of separation is shifted toward the large particle size, andinversely by reducing the speed of the gas stream the line of separationis shifted toward the small particle size.

According to a further feature according to the present invention it isalso possible to divide the elongated container in a plurality ofsucceeding parts along the length of the container and to feed into thesucceeding parts gas with increasing speeds in order to displace'theparticles in suspension toward the outlet opening of the container.Likewise, the space in the receptacle above the layer of particles insuspension may be divided by partitions in corresponding compartments,which partitions end short of the bottom of the container so that thelayer of particles in suspension above the bottom is not divided by thepartitions. The compartments thus formed above the layer are isolatedfrom each other and gas streams of different speeds may thus actindependent from each other. The use of gas streams with increasingspeed is especially advantageous since in the first compartment the gasstream will act on the finest parts of the pulverulent material whichare thus carried away, while the particles carried away in the followingcompartments where the speed of the gas stream is increased further andfurther are less and less small so that in this way a progressiveseparation of the small size particles from the pulverulent materialwill take place. It is likewise possible to collect the different finefractions thus obtained in the various compartments separately from eachother.

The novel features which are considered as characteristic for theinvention are set forth in particular in the appended claims. Theinvention itself, however, both as to its construction and its method ofoperation, together with additional objects and advantages thereof, willbe best understood from the following description of specificembodiments when read in connection with the accompanying drawings.

Brief description of the drawing FIG. 1 is a schematic, partly sectionedside view of an apparatus according to the present invention;

FIG. 2 is a top view of the apparatus shown in FIG. 1;

FIG. 3 illustrates a plurality of classification curves of a pulverulentmineral which are obtained with the process according to the presentinvention;

FIG. 4 illustrates a curve of division obtained from the curves of FIG.3; and

FIG. 5 illustrates another curve of division obtained from the curves ofFIG. 3.

Description 0 the preferred embodiments Referring now to the drawings,and more specifically to FIGS. 1 and 2 of the same, it will be seen thatthe apparatus for carrying out the method according to the presentinvention may comprise a hopper 1 which may be filled with ground ironore having a grain size of for instance 0-3 mm. and from which theground ore is fed by means of a feed screw 3 and a conduit 4 into oneend of a receptacle 2 in form of an elongated rectangle, whichreceptacle may be formed from sheet metal.

The receptacle 2 may be divided into two compartments 5 and 6 separatedfrom each other by a partition 7, the lower edge of which is spaced fromthe bottom of the receptacle so as to form an opening 8. The bottom ofthe receptacle 2 is formed by a grate 9 constituted by two superimposedperforated metal sheets the openings of which are slightly displaced, inone direction from each other as clearly shown in FIG. I.

A blower 10 blows air through air passages 11a, 11b, 11c and 11aarranged beneath grate 9 and obliquely inclined with respect thereto.The blower 10 communicates with the air passages through a commonconduit 12 and branch conduits 13a, 13b, 13c and 13d. The latter arerespectively provided with Venturis 14a, 14b, 14c and 14d which permitto measure the amount of air passing into the air passages and withregulating valves 15a, 15b, 15c and 15d. A conduit 16 branched olf fromthe common conduit 12 and provided with a valve 17 peirnits to send alateral air stream into the compartment 5 in the region where theconduit 4 communicates with the latter. The pulverulent materialintroduced into the compartment 5 through the conduit 4 is thusimmediately dispersed by the lateral air currents coming from theconduit 16. The dust covering the granular particles is thus easilystripped therefrom, which action is further enhanced by a pair ofdeflectors or batfie plates 18 and 19 which will cause formation of eddycurrents at the entrance end of the receptacle and which will increasethe dust removal action.

A plurality of air streams are fed into the compartments 5 and 6 throughthe perforated bottom 9 of the receptacle and through the air passages11a, 11b, 11c, and 11d. The speeds of the air streams passing throughthe aforementioned passages are regulated by means of the regulatingvalves 15a, 15b, 15c, and 15d in such a manner that the air streams formwith the large size particles a turbulent bed of suspension 20 and sothat the finest particles are carried upwardly in the compartments and6. The regulating valves a and 15b may be adjusted in such a manner thatthe air streams entering into the compartment 5 have the same speed andthe regulating valves 15c and 15d may be adjusted in such a manner thatthe air streams entering into the compartment 6 are likewise of the samespeed, which is however superior to the speed of the air streamsentering the compartment 5. On the other hand, it is also possible toadjust each of the regulating valves in such manner that the speeds ofthe air streams passing through the passages 11a, 11b, 11c and 11d areall different from each other and so that the air speeds increase fromthe passage 11a towards the passage 11d, that is in direction ofdisplacement of the particles in suspension.

Due to the horizontal component of the air streams resulting from theinclination of the air passages with respect to the grate formed by thetwo perforated sheet metal members the openings thereof are displacedwith respect to each other in the manner so that the air streams inpassing therethrough can continue their oblique path, the granularparticles in suspension of the bed will follow a horizontal pathindicated by the arrow a. The particles which thus circulate along thevery extended passage are subjected during their passage to the actionof upwardly directed gas streams passing through the bottom of thereceptacle.

The finest particles are thus lifted toward the top of the receptacleand discharged therefrom. Due to a certain pseudo-viscosity of the bedof particles in suspension, certain particles having a dimensionsuperior to the dimension of the desired dividing line are likewiseupwardly projected. The whole bed represents a turbulent mass withprojections of particles extending upwardly. Therefore, the height ofthe compartments 5 and 6 is preferably 20 times the height of the bed ofparticles in suspension so that the large particles which are upwardlyprojected out of the bed will fall downwardly again into the latter tobe discharged with the other large size particles of the turbulent bedat the discharge opening 22. In this way the separation of the particlesaccording to grain size is made more precise.

Obstacles in form of transverse baflie plates 21 may be disposed in thebed in suspension in the path of the granular particles in order toincrease the turbulence of the bed.

In this way, only the large size granular particles will arrive at thedischarge opening 22 located at the end of the receptacle 2 which isopposite the end at which the feed conduit 4 communicates with thereceptacle. The height of the discharge opening 22 determines thethickness of the layer in suspension. The particles after passingthrough the discharge opening 22 are collected in a hopper 23 providedwith level indicators 24 and 25 of known construction. The indicator 24controls, in a manner known per se and not illustrated in the drawing,means for discharging the material from the hopper 23 onto atransporting band 26 when the material in the hopper 23 reaches an upperlevel, while the indicator 25 controls a means known in the art and notillustrated which stops discharge of material from the hopper 23 whenthe material in the hopper reaches a minimum level. During normaloperation, that is when the level of the material in the hopper isbetween the maximum and the minimum level, the granular particles aredischarged from the hopper 23 onto a rubber transporting band 26 whichmay be supported by rollers 27 and driven by a motor 28.

The fine particles which are suspended in the upwardly directed airstreams in the compartment 5 are discharged through the conduits 29a and29b into a cyclone separator 30 in which they are separated from the airwhich is evacuated upwardly through the conduit 31, whereas theparticles move downwardly in the cyclone 30 and are collected in acontainer 32. As pointed out above, the more or less small dimensions ofthe particles thus carried away by the air streams are a function of thespeed of the air streams and in this way it is possible to adjust atwill the line of separation by regulating the speed of the air streams.Due to the greater speed of the air streams in the compartment 6,particles of a dimension superior to that carried away in thecompartment 5 are carried by the gas streams through the conduit 33a and33b into the cyclone separation 34, from which the air is upwardlydischarged through the conduits 35 and 31 while the semi-fine particlespass to the bottom of the cyclone 34 and are collected in a container36. The air evacuated through the conduit 31 may contain a very smallamount of extremely fine dust and therefore this air is passed through adust remover of known type, not illustrated in the drawing. Thereceptacle 2 and the elements connected thereto are supported on a frame37 of known construction.

Following is a description of a classification operation carried outaccording to the present invention with the apparatus illustrated inFIGS. 1 and 2.

An iron ore from the region of Droitaumont (France) has been ground in aball mill of known construction. The product discharged from the grinderhad a grain size from 0-3 mm. and its granulometric distribution isrepresented by the curve A of FIG. 3 in which the abscissa scale isgraduated in the lower part in microns and in the upper part inmilimeters.

The thus obtained product has been treated in an apparatus asillustrated in FIGS. 1 and 2, and it is fluidized by air streamsintroduced through the grate 9. The speed of the air streams enteringinto the compartment 5 was 1.4 m./ sec. and the speed of the air streamsentering the compartment 6 was 2.1 m./sec.

The width of the receptacle was made 12.5 cm. and its length 4 m.

The height of the layer in suspension was in the neighborhood of 10 cm.,whereas the height of the compartments 5 and 6 was 2 111.

After classification three granulometric fractions are obtained, that isa first fraction of granular particles evacuated by the band 26, thegranulometric analysis of this first fraction is represented by thecurve B in FIG. 3; a second fraction of semi-fine particles collected inthe container 36, the granulometric analysis of this second fraction isrepresented by the curve C of FIG. 3; and a third fraction of very fineparticles collected in the container 32, and the granulometric analysisof this third fraction is represented by the curve D in FIG. 3.

From the curves A, B, C and D the curve B is derived indicating thedivision between the large and the fine grain sizes which in the presentcase include very fine and semi-(fine grains, which curve is illustratedin FIG. 4. The curve of division is established in the following manner:on the abscissa is entered on a linear scale the probability of aparticle to pass into one of the fractions and on the ordinate areentered, on a logarithmic scale, the dimensions of the particles. Withina narrow range in the neighborhood of the dimension considered theprobability of passage of the particles in one of the fractions is thesame with respect to the respective weight of this fraction in themixture, and in practice this narrow range will be comprised between twosuccessive mesh sizes of a sieve, for example between mesh sizes of 400to 500 microns when the dimension considered is 450 microns.

The calculation of one point of the curve E is made in the followingmanner:

Assuming the percentage in weight of the fraction of large grain size isG% in the original mixtures and the proportion in weight of the fractionof small grain size is F%.

Assuming further that A% is the proportion in weight at a narrow rangein the neighborhood of the dimension considered of the fraction of largeparticles and a% is the proportion in weight of the fraction of fineparticles at the same narrow range.

The probability of a particle to pass into the fine fraction for thedimension considered will be, expressed in percentage:

aF aF-l-AG and the probability to pass into the granular fraction willbe:

AG AG+aF If, as in the above-mentioned example three fractions arepresent, the fine fraction may be considered as the sum of the fractionsformed by the semi-fine and fine particles. In this case, theprobability of a particle to pass into the fine division will be:

and the probability of a particle to pass into the granular fractionwill be:

wherein a and a respectively represent the weight proportions of theparticles within a narrow range in the neighborhood of the dimension inthe fraction of semi fine and fine particles and F and F represent theproportions in weight of the semi-fine and fine particles in theoriginal mixture.

From the pulverulent material having grain sizes from -3 mm. of theabove-mentioned sample the following results have been obtained:

The fraction of the granular particles (curve B of FIG. 3) represented52% of the total Weight of the raw product (G=52% the fraction of thesemi-fine particles (curve C) represented 24% of the total weight (F=24%) and the fraction of the fine particles (curve D) representedlikewise 24% of the total weight (F =24%).

In order to establish for instance the probability of a particle of adimension around 450 microns (between 400 and 500 microns) to pass intothe fine fraction constituted by the semi-fine and fine particlesassuming that:

A=%; a =0.5%; and a =0.2% one obtains:

In a similar manner the probability of this particle passing into thegranular fraction is:

in other words, the probability of a particle the dimension of which isin the neighborhood of 450 microns to pass into the granular section is97.9% as indicated by the point p in the division curve E shown in FIG.4. In the same manner the points P P etc. are established and the locusof the points P P forms the curve E.

The dimension D of a particle of which the chances of being classifiedwith the granular particles or with the fine particles is equal iscalled the dimension of separation. In the curve B the dimension ofseparation is indicated by the ordinate of the point M (270 microns) andthe abscissa of which is 50% The quality of the separation can beevaluated by considering the central part of the curve E; theimperfection defined by the number:

marks this quality.

which is an excellent value which favorably compares with resultsobtained with processes known in the art.

In the same manner the curve of division G (FIG. 5) is established forthe semi-fine particles (curve C of FIG. 3) and the fine particles(curve D), by applying the formulas and the imperfection is in this case0.25 which is still an excellent value.

The process according to the present invention is particularlyapplicable to classify pulverulent iron ore into distinct granulometricfractions before subjecting the fractions to different treatments. Inthe above mentioned example the mean iron content of the pulverulentmaterial to be classified was 31%. In the large grain size fraction, theiron content was reduced to 26% and this fraction is subsequentlysubjected to an enrichment process by magnetizing roasting followed by amagnetic separation at high intensity; in the semi-fine fraction theiron content was 35% and this fraction may be treated with greatefficiency because it does not contain ultra-fine material in a magneticseparator of great intensity, whereas the fine fraction, in which theiron content was up to 38%, did not require a complementary enrichmentprocess.

An additional advantage of the present invention resides in the factthat the novel apparatus can be extended as desired whereby it is onlynecessary to elongate the fluidized bed in order to increase theproduction. This can be carried out in a very simple manner byprolonging the receptacle by addition of further compartments. Theproduction, that is the quantity of the pulverulent material treated pertime unit, is proportional to the surface of the grate, and duringexperiments carried out by the inventor, it has been found that 7 tonsof ground iron ore may be treated per hour for each square meter ofgrill.

A further advantage of the present invention is that a relatively smallgas pressure is necessary for carrying out the classification of theproduct. In fact, only a pressure of 300 mm. Water column is required toobtain a gas speed of 2 -m./ sec. From this follows that the electricenergy utilized for driving the blower is relatively small and theelectric energy is about 1 kilowatt/hr. per ton of pulverized iron oreto be treated.

Furthermore, the process according to the present invention leads to aperfect classification of the pulverulent material, and the imperfectionof the classification, as defined above, is about 0.12.

It will be understood that each of the elements described above, or twoor more together, may also find a useful application in other types ofpneumatic classification of pulverulent materials differing from thetypes described above.

While the invention has been illustrated and described as embodied inpneumatic classification of pulverulent iron ore or other materials, itis not intended to be limited to the details shown, since variousmodifications and structural changes may be made without departing inany way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can by applying current knowledgereadily adapt it for various applications without omitting featuresthat, from the standpoint of prior art, fairly COnStitute essentialcharacteristics of the generic or specific aspects of this inventionand, therefore, such adaptations should and are intended to becomprehended within the meaning and range of equivalence of thefollowing claims.

What is claimed as new and desired to be protected by Letters Patent isset forth in the appended claims.

We claim:

1. A process of pneumatically classifying pulverulent materialcomprising the steps of forming an elongated bed of pulverulent materialin suspension of a given height and of a length at least six times itswidth by feeding into the pulverulent material upwardly directed gaswith a speed at least suflicient to keep the largest particles ofpulverulent material in suspension and less than the speed necessary tolift the largest particles out of the bed in suspension, but greaterthan the lifting speed of the smallest particles; continuously feeding astream of pulverulent material to be classified into one end of theelongated bed in suspension; continuously withdrawing the largestparticles at the opposite end of the bed at the level of the top face ofsaid bed so that the large particles in the bed will move horizontallyfrom the one to the other end thereof; maintaining above said bed insuspension a free space at least eight times the height of said bed intowhich the finest particles of the pulverulent material are carried bysaid upwardly directed gas streams; evacuating said gas streams with thefinest particles suspended therein from said space; and separating saidfinest particles from said gas stream.

2. A process as defined in claim 1, wherein at least two upwardlydirected streams of gas are introduced in to said bed with one of saidstreams closer to said other end of said bed having a higher speed thanthe other.

3. A method as defined in claim 1, and including the step of feedinginto said stream of pulverulent material fed at said one end into saidbed a lateral stream of gas so as to enhance removal of dust clinging tosaid largest particles and to enhance movement of said large particlesfrom said one to said other end of said bed.

4. A method as defined in claim 1, wherein said upwardly directed gas isfed into said bed at an angle inclined in a direction toward the otherend of said bed to the vertical to enhance thereby displacement of thelargest particles from said one to said other end of said bed. 1

5. A process as defined in claim 1, wherein said pulverulent material tobe classified is pulverulent ore, and wherein the gas is fed with aspeed of 1-2 meters per second into said bed of pulverulent material.

6. A process as defined in claim 1, wherein the height of said bed doesnot exceed thirty centimeters.

7. An apparatus for pneumatically classifying pulverulent materialcomprising, in combination, an elongated receptacle having at least alength six times its width and a perforated bottom wall; meanscommunicating at one end of said receptacle and at a given distanceabove said bottom wall with the interior of said receptacle forcontinuously feeding a stream of pulverulent material to be classifiedthereinto; means for feeding a gas under pressure in upward directionand at such speed through said perforated bottom wall so as to form fromthe largest particles of the pulverulent material fed into thereceptacle a fluidized bed above said bottom wall; means for dischargingthe largest particles from the other end of said receptacle at adistance from said bottom wall substantially equal to said givendistance so that a fluidized bed of a height substantially equal to saidgiven distance will be maintained in said receptacle, said receptaclehaving a height of at least nine times 12' said given distance; andmeans communicating with the top portion of said receptacle fordischarging the gas carrying the finest particles of pulverulentmaterial from the interior of said receptacle.

8. An apparatus as defined in claim 7 and including means for separatingsaid finest particles from said gas.

9. An apparatus as defined in claim 8, wherein said separating meanscomprise at least one cyclone.

10. An apparatus as defined in claim 7, wherein said means for feedinggas in upward direction through said perforated bottom wall into theinterior of said receptacle comprise at least two passages one of whichcommunicates at its inner end with the interior of said receptaclecloser to said other end thereof than the other of said two passages,and means for feeding gas of different speeds through said passages.

11. An apparatus as defined in claim 10, wherein said passagescommunicate at the outer ends thereof with each other, and wherein saidmeans for feeding gas at different speeds through said passages comprisea single blower means communicating with said outer ends of saidpassages, and regulating means located in each of said passages forregulating the speed of the gas passing therethrough so that the speedof the gas entering through the inner end of said one passage into saidreceptacle is greater than that entering through said inner end of saidother passage.

12. An apparatus as defined in claim 7 and including means forintroducing into said stream of pulverulent material as it is fed intosaid receptacle a stream of gas under pressure in direction transverseto said stream of pulverulent material to enhance separation of largesize particles of said pulverulent material from said small sizeparticles at the point of introducing said particles into saidreceptacle.

13. An apparatus as defined in claim 7, wherein said means for feedinggas under pressure through said perforated bottom wall extend inclinedtoward said other end of said receptacle to the vertical so as toenhance flow of the particles in said bed from said one to said otherend of said receptacle.

14. An apparatus as defined in claim 7 and including a partitionextending from the top of said receptacle toward but short of the bottomwall and dividing said receptacle into two compartments communicatingwith each other adjacent said bottom wall, said means for feeding gasunder pressure into said receptacle comprising at least two gas passagemeans respectively communicating through said perforated bottom wallwith said compartments, and said means for discharging the gas carryingthe finest particles communicate with a top portion of each compartment.

15. An apparatus as defined in claim 14, wherein said gas dischargemeans comprise a pair of separate conduits respectively communicating atthe inner ends with top portions of the two compartment, and cyclonemeans communicating with the outer ends of said conduits.

16. An apparatus as defined in claim 7 and including at least one baffleplate extending transverse to the elongation of said receptacle at leastpartly through said bed of large size particles in suspension.

References Cited UNITED STATES PATENTS 2,269,307 1/ 1942 Dickerson209-467 2,586,818 2/ 2 Harms 209-474 2,743,817 5/ 1956 Musgrave 209-4742,774,661 12/1956 White 209-474 X 2,865,504 12/1958 Zubrzycki 209-138FRANK W. LU'ITER, Primary Examiner.

US. Cl. X.R.

